This year's hurricane activity is high but not unusual, according to UA scientists Thomas Galarneau and Xubin Zeng. What's unusual is that two Category 4 storms hit the U.S.

Two Things to Know About the Hurricane Season of 2017

UA researchers have been busy forecasting and studying this year's highly active hurricane season — and it isn't over yet. The season runs through November and could include even more destructive storms.

Click here[2] to read a UANews story from July 2016 about the work of Galarneau and Zeng.

Thomas Galarneau (left) and Xubin Zeng look at the 2017 hurricane season from two different perspectives. Galanreau studies the short, one-to-two-week windows of activity, while Zeng takes a long-term approach to research and forecasting. (Photo: Stacy Pigott/UA News)

This map, last updated on Aug. 31, is a summary of the tracks of all the named tropical cyclones in 2017. (Photo: National Hurricane Center)

A computer model simulation of outgoing longwave radiation (OLR) for Hurricane Harvey at 5 p.m. CDT (22:00 Greenwich Mean Time) on Aug. 25. Lower values of OLR are shown in shades of blue and represent deep thunderstorm clouds. Higher values of OLR, shown in shades of red, represent clear skies. The computer model simulation was produced by Galarneau and Zeng at the UA using the Weather Research and Forecasting model.

On Aug. 17, a tropical storm, the eighth of the year, formed to the east of the Lesser Antilles in the Atlantic Ocean. A week later, the storm intensified and was upgraded to hurricane status as it began to threaten the Texas coast.

By Aug. 25, when it hit ground south of Houston, Hurricane Harvey was a household name. News outlets covered the storm as it devastated Houston and the surrounding areas, then moved on to Louisiana before finally dissipating on Sept. 2.

By then, a new threat was on the horizon, as meteorologists were tracking Hurricane Irma, which was expected to hit Florida hard — and did. Irma wasn't alone, however, as Hurricanes Jose and Katia also made their way through the Atlantic basin, including the Gulf of Mexico and Caribbean Sea, in early September.

For hurricane forecasters and researchers at the University of Arizona, the high hurricane activity is one they expected and have been studying with great interest. Two UA faculty, Thomas Galarneau, assistant professor in the UA's Department of Hydrology and Atmospheric Sciences, and Xubin Zeng, Agnese N. Haury Chair in Environment, director of the UA's Climate Dynamics and Hydrometeorology Center and professor of atmospheric science, bring interesting perspective to this year's hurricane season.

Two things you need to know about this year's hurricanes:

1. This year's activity is high — but not unusual.

"These types of events, where you have multiple hurricanes in the Atlantic, happen relatively frequently," Galarneau said. "It has happened before, so it's not that unusual, but this year these hurricanes had high socioeconomic impacts over the U.S. We had Harvey and then we had Irma, and then Jose — which will likely recurve out to sea — and then Katia, as well. It's a situation where these multiple cyclones had very high impacts, which brings a lot of attention to these types of events."

UA scientists have been expecting an active hurricane season based on results of a forecasting model developed in 2015 by Zeng, Kyle Davis and Elizabeth Ritchie. On June 10, the model predicted 11 hurricanes for the 2017 Atlantic hurricane season, six of which would be major (Category 3 or above). This is the first year the UA forecast has included major hurricanes.

"When we issued our forecast in early June, our number was so high and we were nervous," Zeng said. "Our forecasted number for hurricanes was 11; the average from the four other centers was less than seven. We were outliers, basically. A couple of months later, we didn't change our forecast, but everyone else moved their numbers up to be closer to us.

"Today, there have already been six hurricanes. We forecasted six major hurricanes; the average was two. Now we already have three."

A major hurricane is one ranked as a Category 3 or higher, meaning it has a maximum wind speed of at least 111 mph. Hurricane Harvey reached Category 4 status when it made landfall near Rockport, Texas, on Aug. 25. Hurricane Katia, which formed in the Bay of Campeche, looped around and made landfall in southern Mexico on Sept. 9.

Hurricane Irma reached Category 5 status during its trek through Puerto Rico, the Dominican Republic, the Bahamas and Cuba before being downgraded to a tropical storm in Florida on Sept. 11. Hurricane Jose, still active in the Atlantic east of the Bahamas, reached Category 5 before being downgraded to Category 2 on the morning of Sept. 11.

Those four hurricanes were preceded by Hurricane Franklin, which made landfall on Aug. 10 as a Category 1 hurricane in Veracruz, Mexico, and Hurricane Gert, a Category 2 hurricane that remained well off the East Coast of the U.S. from Aug. 13-17.

2. Hurricane activity in the Atlantic basin is following an alphabetical trend.

Of the six hurricanes so far this year, three — Irma, Jose and Katia — were active in the Atlantic basin at the same time. While the event was rare, it isn't unprecedented.

"It did occur in the past, in 2010," said Zeng, referring to Hurricanes Igor, Julia and Karl. "In 1998, there were four hurricanes at the same time, even though three of them never reached the continent. Not many people paid much attention."

Of those four 1998 hurricanes — Ivan, Jeanne, Karl and Lisa — only Jeanne brushed land, in the Cape Verde Islands, while the others remained over water.

"An interesting fact is all those events happened with the same alphabetical order," Zeng said. "Keeping in mind this year it's I-J K, 2010 is the same. It was I-J-K. And 1998 was I-J-K-L. From the beginning, the number of storms is quite similar."

Tropical storms are named in alphabetical order, starting with the first one of the year. In each year that the Atlantic has seen three or more active hurricanes at the same time, the streak has started with the "I" storm, the ninth tropical storm of the season.

"This year is very different, because two Category 4 hurricanes hitting the U.S. is really rare," Zeng said. "To have three hurricanes at the same time is not as rare — it happens about once every decade or so."

"These types of events are what scientists have recently called multiple tropical cyclone events, where there were several systems lined up across the Atlantic," Galarneau added. "The sea surface temperatures are warm and that helps moisten the atmosphere. Another factor is we have very weak vertical wind shear, which is the change in horizontal wind speed and direction with height. That's been fairly weak, which is good for hurricanes. Those conditions have come together, and we're in a situation where three tropical storms in a row have moved westward from North Africa into favorable conditions over the Atlantic Ocean and developed into hurricanes. It has happened before. The issue this year is that these storms have impacted the U.S. directly."

The hurricane season runs from June 1 to Nov. 30, so it is far from over. If the UA's forecasting model remains accurate, there could be several more hurricanes to come, including more major hurricanes. Whether future hurricanes will remain over water or threaten the U.S. coast again is an unknown variable.

"The current numerical models are suggesting the possibility that the next storm system moving west over the Atlantic Ocean could develop, but it's way, way, way too early to say if that would affect the U.S.," Galarneau said. "There's a lot of time left in the hurricane season, so it remains to be seen what will happen."

"Forecasting the future is always very dangerous," Zeng said. "We have done well in the past. This year it looks like we're in good shape, but you never know. We don't know how good our forecasting will be, because this is far from the end of the season."